Sensing method and device

ABSTRACT

Devices and methods for sensing the charge condition of a mixed stream of aggregant and aqueous dispersion of charged particles are based on sensing changes in the potential of an electrode contacting the stream. Sensing the changes in potential permits better control of aggregation in raw water and effluent treatments.

This invention is concerned with a method of following the influence ofa coagulant and/or flocculant (hereafter referred to as an "aggregant")on a stream of an aqueous dispersion of charged solid or liquidparticles and a device suitable as one component of a control loopprocess governing dosing of the aggregant.

THE FIELD OF THE INVENTION

The process of aggregating aqueous dispersions of charged particles iswidespread in many industries, including, for example, the treatment ofraw water in making drinking water, effluent and sludge treatment andpaper-making.

The stability of an aqueous dispersion depends on the electrical doublelayers which exist at the surfaces of the particles and extend into theliquid phase. Coagulation is caused by changes in the structure of thedouble layers.

The most widely used method of coagulating a disperse system, is to addcolloids having an effective change opposite in sign to that carried atthe surface of the disperse phase: thus "solutions" of alum or ferricsalts (in fact dispersions of colloidal cationic hydroxylic polymers ofaluminum or of ferric iron) are effective coagulating agents for naturaldispersions such as raw water, sludge and effluent and some man-madedispersions such as pulp dispersions used in paper-making, all of whichare anionic.

Maintaining ideal coagulation conditions in a stream has never beeneasy. The jaw test which is in widespread use is at best an uncertainguide; the process is so lengthy that by the time the result isavailable, the condition of the stream may have changed so significantlythat the test is no longer appropriate to determine the action to betaken.

Flocculation involves the interaction of a colloidal solution of aflocculant with the disperse phase.

DISCUSSION OF THE PRIOR ART

The first, and hitherto the only, step towards automating themaintenance of a chosen aggregation condition in a mixed stream of anaqueous dispersion and an aggregant, was to use devices utilising theelectrokinetic phenomenon of streaming current. An account of thecompetence of these devices generally known as Streaming CurrentDetectors, or SCDs, is contained in the specification of European PatentApplication 88901621.8 SCDs have a shared essential characteristic,namely that samples of the stream are caused to flow back and forethrough a capillary, limiting the in-line application of SCDs tomonitoring the treatment of very dilute dispersions of colloids of smallparticle size, like some raw waters in making drinking water. The rangeof applications of SCDs was increased by the discovery that the chargedcondition of the filtered mixed stream was identical with that of theunfiltered mixed stream, but that introduced the addition step offiltration and a delay in the analysis.

The addition of alum to a pulp dispersion was thought to be controllableindirectly by the acidity of the mixed stream using a pH meter with ahydrogen-selective electrode, but the method took insufficient accountof the accumulation of the sulphuric acid in the mill's recycled waterand has been displaced by SCDs used off-line.

There is a need for an aggregation control method able to operatein-line on such aqueous dispersions as sewage, effluent, pulpdispersions and raw waters of all kinds.

One object of the invention is to provide such an in-line method and adevice for employment in the method.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided apotentiometric method for sensing the charge of a mixed stream of anaggregant and an aqueous dispersion, suitable for use in an aggregationcontrol loop process, which comprises flowing the mixed stream incontact with a metallic indicator electrode, measuring the potentialdifference between the indicator electrode and a second electrode ofsubstantially stable potential different from that of the indicatorelectrode, whilst preventing any substantial electrical current fromflowing between the electrodes.

For the purposes of this specification, aggregation means the process ofcoagulation, or that of flocculation.

Desirably the potential difference is measured with a device formeasuring potential which allows only negligible electric current (e.g.less than 10⁻¹⁰ amps) to flow between the electrodes.

The second electrode may be a reference electrode, or (and preferably,when the mixed stream is earthed) an earthed electrode or an earthedconnection to the reference terminal of the device for measuringpotential.

It is common practice to classify electrodes as either metallicelectrodes or membrane electrodes; a carbon electrode is classed as ametallic electrode.

The method may be carried out using a direct-reading pH meter orpotentiometer equipped with the metallic indicator electrode. Suchmeters, hitherto, have been considered to respond solely to solute ions,their reactions and concentrations and are generally equipped withion-specific membrane electrodes. No such electrochemical cell has beenproposed for use in directly discriminating the reaction of an insolublecolloid with another oppositely charged colloid or colloidal solution.Earthing one electrode of a pH meter has never been proposed butsurprisingly if such earthing is effected and an appropriate choice ofindicator electrode is made, highly effective aggregant addition controlof an earthed mixed stream can be achieved using such a pH meter.

There is no accepted theory which explains the present method. Theinventors would not wish to be held to either of the following possibleexplanations, but is appears to them that the negatively charged surfaceof a cathode is a bite exerting electrostatic attraction on the freecolloidal cationic aggregant which appears when the interaction of thecolloids is substantially complete: there may follow a close associationof the aggregant and the electrode surface, or a compression of theHelmholtz layer. Alternatively, an anionic aggregant (for example thecopolyacrylic acids used in paper-making) may act as a ligand for metalions released at the anode. Any such process would affect the potentialof the electrode and the potential difference reading.

The interaction of an electrode surface and the aggregant will depend onthe nature of the aggregant, the relative proportions of disperse phaseand aggregant in the mixed stream, as well as on the electrodepotential, electrode material, pH, temperature and flow rate. The latterthree variables may be compensated, or so reduced in consequence that nocompensation is required, for example when the aqueous dispersion issubject to pH, temperature or flow control.

Thus the method is directly sensitive to the charge condition of themixed stream. No sensor, apart from SCDs, has been suggested for thisuse and none, including the SCDs, has been capable of in-line use inhigh solid streams such as sewage, effluent and pulp dispersions.

The indicator electrode should have no particular ion-specificity and astable surface substantially chemically inert to the electrolytes in themixed stream. Suitable metallic electrodes include stainless steel, gunmetal bronze, carbon, gold, lead, platinum and silver, of which thefirst four have proven to be the most widely useful in the various mixedstreams to which the method and device may be applied: the others areselectively useful, depending on the chemical nature of the stream. Forexample, lead and silver are changed at their surfaces by streams whichcontain hydrogen sulphide (sewage effluents) and platinum, from itssensitivity to hydrogen ion, is not preferred in streams with variableacidity.

Specifically excluded as indicator electrodes are membrane andion-selective electrodes such as hydrogen electrodes used in pHmeasurement, where the response to a particular ion is dominant andobscures whatever response such as electrode may have to the chargecondition of the stream.

According to a further aspect of the invention a device for sensing thecourse of the aggregation of an aqueous dispersion of charged particlesin an aggregation control loop process, comprises a galvanic cell havinga metallic indicator electrode without any particular ion-specificityand a second electrode, the electrolyte of the cell being a mixed streamof the dispersion and an aggregant for the charged particles and beingin contact with the indicator electrode, and a means for sensing thepotential difference appearing between the electrodes.

The potential difference of the galvanic cell may arise spontaneouslyfrom the difference in the potentials of the two electrodes, or thepotential difference may be impressed on the cell by an external sourceof stable e.m.f. bridging the electrodes. In the latter case theelectrodes may be of similar composition.

In a cell with an impressed potential difference, preferably bothelectrodes are metallic.

In a cell in which the potential difference arises spontaneously, thesecond electrode acts as a reference electrode developing asubstantially stable potential so that the changes in the potential ofthe indicator electrode may be recognised by changes in the cell e.m.f..The known reference electrodes comprising half cells of a metallicelectrode in a standard solution, for example the silver/silverchloride(solid)/KCal (solution) composite are useful, as are asilver/silver chloride solid electrode with no surrounding standardsolution, and, particularly for use in the aggregation of particles ineffluent and sewage, a silver/silver sulphide electrode.

Whilst the indicator electrode must contact the mixed stream, it is notalways necessary that the second electrode should do so. If there is noextraneous, significant, variable electrical field, such as would form aground loop affecting the potential difference, and the mixed stream isearthed, it is sufficient that the second electrode is an earthedconnection from the voltmeter directly to earth or to a metal conduitcarrying the stream. Preferably such earthed connection is the anode.

The indicator electrode or both electrodes may be housed in a conduitwhich determines the path of the mixed stream, or they may be mountedexternally on a body for immersion in the mixed stream.

The potential difference is desirably measured by a high impedancevoltmeter drawing insufficient electric current to affect the potentialdifference between the electrodes--available electronic voltmetersoperating with a current of from 10⁻¹² to 10⁻¹⁴ amperes are ideal,preferably the voltmeter incorporates electronic amplification toproduce a signal which can be displayed in a digital and/or analog form.The conductivity of the mixed stream is of no consequence to theoperation of a sensor cell comprising such a voltmeter.

There is, in the vicinity of aggregation, for each substantial change inthe electrical double layers of the particles, a corresponding andsubstantially unique potential difference which is sensed by the methodand device of this invention. If there is efficient aggregation of theparticles, the corresponding measured potential difference can be usedas a datum signal for the preservation of that condition despite changesin the concentration of particles and/or the charges they carry.

Furthermore, the indicator electrode may be situated no more than 30seconds in real time downstream of an aggregant dosing point: this shortresponse time is valuable to close control and compares with 40 minutesreal time delay for a jar test and 5 to 8 minutes delay for an SCDoperating on a filtrate derived from the stream.

It should be understood that a control loop method based on the methodof the invention does not determine the best coagulation conditions, butwill preserve a selected coagulation condition established by othermeans by preserving the datum signal generated by the selectedcondition.

The method of the invention may discriminate the effect of a trace, sayless than 1 part per million, of coagulants such as hydroxylic polymersof aluminium and of ferric iron in the vicinity of efficientcoagulation, and flocculants such as synthetic polycations. The methodmay also, in similar circumstances, discriminate traces of polyanionicsubstances, for example copolyacrylic acids and polyphosphates, used inpaper-making.

When it is intended that the indicator electrode and a referenceelectrode both contact the mixed stream, the electrodes may be disposedrelatively to each other transversely or longitudinally of the path ofthe stream.

A range of useful datum potential differences is available from thechoices of cell conformations, electrode materials and/or externallyapplied e.m.f..

Monitoring aqueous streams in conduits from capillaries to pipes ofseveral hundred millimeters diameter would encompass most practicalapplications, but no limit has been found on the size of the conduit orthe rate of flow of the liquid.

In dealing with some mixed streams, it is seldom possible to use asampling pump to abstract a side stream from the very large works' pipes(frequently 700 mm diameter) to feed to the sensor cell, firstly becausesuch mixed streams are prone to block the pump and secondly such fastacting pumps wear quickly from contact with the abrasive material suchmixed streams frequently contain. In these circumstances and the absenceof interfering ground loops, the indicator electrode may be let into thepipe (electrically insulated from the pipe if it is metallic) and thesecond electrode may be a connection between the voltmeter and earth orto the pipe as appropriate.

The surface of an electrode in contact with some mixed streams, will bekept in working condition by abrasion from components in the mixedstream. In other cases the surface may become fouled and then must becleaned by mechanical abrasion, electrolytically, or ultrasonically. Theor each electrode may be placed or shaped to reduce the incidence offouling--flush fitting in the inner wall of the conduit, cambering, orstreamlining.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example, withreference to the accompanying drawings, in which:

FIGS. 1 to 5 are schematic cross-sectional representations of a seriesof devices according to the invention showing variations in a measuringcell.

FIG. 6 is a cross-sectional representation of a cell with externalelectrodes.

FIG. 7 is a reproduction of a chart recording of an inefficientlycoagulated effluent stream monitored by a device according to theinvention.

FIG. 8 is a reproduction of a chart recording of an effluent streamdosed in accordance with the output of a device according to theinvention.

FIGS. 9 and 10 show alternative arrangements of electrodes in sensingdevices according to the invention, and

FIG. 11 shows a portable cell for use in the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a length of horizontal works' pipe 1 (through whichflows the grounded works' effluent) shown sectioned on a verticaldiameter, has a port 2 closed by an insulating rubber collar 3surrounding a silver indicator electrode 4. An earthed connection 5serves as a reference electrode. The electrodes 4 and 5 are bridged by ahigh impedance voltameter 6 via screened cables 7a and 7b respectively.The output of the voltameter is connected to a signal processor 8 fromwhich a control signal is relayed to a chemical dosing pump 9 via acontrol line 10.

Apparatus according to FIG. 1 was installed at an effluent works using acommercial polyquaternaryammonium salt as flocculant. The works' chemistestablished the dose of flocculant to give satisfactory flocculation andthe datum reading of the voltmeter 6 was established as 97. The controlloop was put in control of the flocculant dosing pump 9 to preserve adatum reading of 97 and through three weeks of continuous operation, thequality of flocculation remained constant without need of recalibrationof the datum reading. During a period of about two hours, the dispersionwas of low solids content and the dosing pump stalled under the lowrevolutions demanded by the automatic control. Manual control wasinstituted to maintain the datum reading. The quality of theflocculation continued satisfactory and was later returned to automaticcontrol. Over the three week period a 25% saving of flocculant waslogged and the plant had not previously enjoyed such a sustained periodof acceptable flocculation.

Referring to FIG. 2, a polypropylene pipe 11 shown in longitudinalvertical cross-section, conducts an ungrounded mixed stream. A port 12in the pipe is sealed by a collar 13 of synthetic rubber surrounding astainless steel indicator electrode 14. A reference electrode 15 ofsilver/silver sulphide is housed within an insulating collar 13a sealingport 12a. The electrodes 14 and 15 are bridged by a high impedancevoltameter 16 via screened cables 17a and 17b and the output of thevoltmeter is connected to a signal processor 18. The processor 18 isused to control the addition of aggregant to the stream flowing in pipe11 upstream of the region shown.

Referring to FIG. 3, a right-sectioned cylindrical conduit 21, ofpolyvinyl chloride, shown end-on, has a screwed-in indicator electrode24 and a reference electrode 25, both of stainless steel. The electrodesare bridged firstly by a high impedance voltmeter 26 through cables 27aand 27b, and secondly by a battery B having an e.m.f. of 2 voltsconnected in series with a resistor R of 0.4 megohms. The voltmeter 26is connected to a signal processor 28. The processor 28 is used tocontrol the addition of aggregant to the stream flowing in pipe 21.

FIG. 4 shows an alternative arrangement to FIG. 2, the electrodes 14 and15 being opposite each other and differently mounted in the tube.

Referring to FIG. 5, the numbered items 11 to 18 have the meaning givento items 11 to 18 in FIG. 4, but an additional electrode 19 is setupstream of the other electrodes and a screened cable 20 connectselectrodes 14 and 19. The third electrode 19 increased the potentialdifference available from the cell and ay make amplifying of the outputsignal unnecessary.

In a trial at a slaughterhouse, unfiltered effluent from theslaughterhouse was flowed through the conduit 11 of FIG. 5 (diameter 5centimeters) and the characteristic reading was exhibited on a digitaldisplay 18A. FIG. 7 shows the record of the display over a twelve hourperiod with dosing of a suitable coagulant determined under manualcontrol guided by jar tests. During this period the coagulation was moreoften poor than good. FIG. 8 shows the record of continuous manualadjustment of the coagulant dose guided by the digital display 18A. Goodcoagulation was known to occur at a reading of 450 and the dosingadjustments were undertaken to seek to maintain this figure. Thecoagulation was of consistently good quality throughout this period andit was found that there was a display band width of from 443 to 455 inwhich this quality was assured. This performance would be guaranteed ifthe output signal of the processor 18 was used to control coagulantdosing in a coagulant dose control loop.

Referring to FIG. 6, a portable cell for use in the method of thepresent invention comprises a T-piece 31 of polyvinyl chloride whichcarries external electrodes 32, 33 and 34 of M316 stainless steel.Electrodes 32 and 33 are connected by screened cable 35 and electrode 34is connected to a cable 36. Cables 35 and 36 issue from the T-piece 31for connection of a high impedance amplifier and signal processor (notshown). This form of device is useful for occasional or permanentexaminations of industrial effluent streams running in otherwiseinaccessible gullies. The device of FIG. 6 has been tested successfullyin a gully 3.7 meters deep in which ran effluent from a paper mill. Thiseffluent had routinely blocked a sampling pump meant to lift acontinuous sample to the filter of a filter/SCD sensing device used formonitoring a parameter of the effluent. The cell shown in FIG. 6 enabledeffective parameter monitoring to be achieved without blockage problems.

Referring to FIG. 9, a conduit 41 of partial frusto-conical shape isshown, housing a larger annular electrode 42 having an internal diameterof 12.5 millimeters, at the wide end of the frusto-conical part and asmaller annular electrode 43 having an internal diameter of 6.25millimeters at the narrow end; each of the electrodes 42 and 43 isequipped with an extension 45 or 46 respectively, which penetrates thewall of the conduit for electrical connection to a voltmeter (notshown).

An advantage of a venturi-shaped zone with the electrodes situated atthe narrowest bore is that there is a self-cleaning effect achieved withthis set-up caused by the acceleration of flow of the liquid as itstreams through the constricted bore.

Referring to FIG. 10, a conduit 61 of polypropylene has a waisted bore67 and houses stainless steel electrodes 62 and 63 with non-conductingspacers 68 and 69 all located at the waist, flush with the boundary ofthe bore. Stainless steel lugs 65 and 66 are secured to the electrodes63 and 62 respectively for connection to other elements of the cell.

FIG. 11 shows a modified form of portable test device for use in themethod of the invention. Hollow tube 71 includes an electrode pair (e.g.such as shown in FIG. 10) which can be dipped into a flowing streamusing handle 72 and hollow support shaft 73. The potential createdacross the electrode pair is sensed by a millivoltmeter 74 via wirespassing through the hollow shaft 73. An earthing spike 75 can be clippedto the shaft 73 when not required, but is used to ground one side of thevoltmeter if a grounded effluent stream is being monitored, in whichcase only the indicator electrode (62) need be connected to the otherside of the voltmeter.

We claim:
 1. A method for sensing the course of an aggregation of anaqueous dispersion of charged particles in an aggregation control loopcomprising a galvanic cell having a metallic indicator electrode withoutany particular ion-specificity and a second electrode, the electrolyteof the cell being a mixed stream of the dispersion and an aggregant forthe charged particles, adding the aggregant to the stream upstream ofthe electrodes and mixing the aggregant into the stream to form themixed stream, maintaining the electrolyte in contact with the indicatorelectrode, sensing a potential difference appearing between theelectrodes, and controlling the addition of the aggregant to maintainthe potential difference within a control range.
 2. A method as claimedin claim 1, in which the second electrode is a reference electrode.
 3. Amethod as claimed in claim 1, in which the second electrode is connectedto earth.
 4. A method as claimed in claim 1, in which the mixed streamis caused to flow in a conduit, and the electrodes are located in theconduit.
 5. A method as claimed in claim 1, in which the electrodes arelocated on a body, and the body is immersed in the stream.
 6. A methodfor monitoring an aggregation control loop process comprising the stepsof:(a) providing a stream of an aqueous dispersion of charged particles;(b) dosing the stream of the aqueous dispersion of charged particleswith an aggregant to produce a mixed stream; (c) providing a referenceelectrode at a selected location having a stable electrical potential;(d) locating an indicator electrode in contact with the mixed stream tomeasure an electrical potential generated by the mixed stream at theindicator electrode; and (e) monitoring the electrical potential betweenthe indicator electrode and the reference electrode to establish aselected electrical potential for the mixed stream.
 7. The methodaccording to claim 6 comprising the step of locating the referenceelectrode in the mixed stream.
 8. The method according to claim 6comprising the steps of providing a conductive pipe for directing themixed stream along a selected path and locating the reference electrodein contact with earth.
 9. The method according to claim 6 wherein themonitoring step is effected by providing a voltmeter that permits only aminimal flow of current to measure the electrical potential between theindicator electrode and the reference electrode.
 10. The methodaccording to claim 9 comprising the step of monitoring the mixed streamusing the voltmeter to detect fluctuations in the electrical potentialbetween the indicator electrode and the reference electrode from theselected electrical potential.
 11. The method according to claim 10comprising the step of adjusting the dosing of the aqueous dispersion ofcharged particles with the aggregant upon detection of fluctuations inthe electrical potential so that the electrical potential between theindicator electrode and the reference electrode returns to the selectedelectrical potential.
 12. An apparatus for monitoring a mixed streamflowing in a conduit comprising:(a) control means positioned along theconduit for dosing an aqueous dispersion of insoluble charged particlesflowing in the conduit with an aggregant thereby forming the mixedstream; and (b) a potentiometer for positioning along the conduitdownstream of said control means, said potentiometer having a metalindicator electrode and a reference electrode for electrical contactwith the mixed stream, said potentiometer providing a measurement of anelectrical potential of the mixed stream, said control means beingresponsive to said measurement to maintain the electrical potential ofthe mixed stream at a selected level by regulating the dosing effectedby said control means.
 13. The apparatus as recited in claim 12comprising a signal processor responsive to said potentiometer forregulating the dosing by said control means so that the electricalpotential of the mixed stream is automatically maintained at theselected level.
 14. The apparatus as recited in claim 12 wherein theconduit has a port downstream of the control means, said indicatorelectrode being mounted in the port, said apparatus comprising aninsulating collar encircling said indicator electrode for insulatingsaid indicator electrode from the conduit and for protecting saidindicator electrode from damage by the mixed stream.
 15. The apparatusas recited in claim 12 wherein the conduit has an annular housingdownstream of said control means, said indicator electrode being annularin shape and dimensioned to engage within the housing of the conduit.16. The apparatus as recited in claim 12 wherein the conduit iselectrically connected to earth and wherein said reference electrode ofsaid potentiometer is electrically connected to earth.
 17. An apparatusfor monitoring a flowing mixed stream comprising:(a) an earthed conduitfor directing the mixed stream along a selected path; (b) control meanspositioned along said conduit for dosing an aqueous dispersion ofinsoluble charged particles flowing in said conduit with an aggregantthereby forming the mixed stream; (c) a potentiometer having a metalindicator electrode positioned within said conduit downstream of saidcontrol means for contacting the mixed stream, said indicator electrodehaving a charged outer surface of like polarity as the charged particlesso that the charged particles in the dispersion are repelled away fromsaid indicator electrode, said potentiometer also having a referenceelectrode for electrically connecting to earth, said potentiometerproviding a measurement of an electrical potential between the mixedstream and earth; and (d) a signal processor responsive to saidpotentiometer for regulating the dosing of said control means so thatthe electrical potential of the mixed stream is automatically maintainedwithin a selected range.
 18. The apparatus as recited in claim 17wherein said conduit includes an interior frustoconical surface toaccelerate the flow of the mixed stream and wherein said indicatorelectrode is positioned in the accelerated flow to allow cleaning ofsaid indicator electrode.
 19. The apparatus as recited in claim 17wherein said conduit includes a waisted bore, said indicator electrodebeing annular in shape and positioned in said waisted bore of saidconduit.